The invention relates to a method for operating an anti-theft system for a motor vehicle with which access to a motor vehicle is enabled only given confirmation of authorization by means of a code generator.
In a known anti-theft system (DE 43 29 697 C2), instead of a mechanical key, the holder of the vehicle carries with him what is referred to as a code generator (for example in the form of a check card) in order to unlock his vehicle. As soon as an activation means, for example a door handle, is actuated on the motor vehicle, the vehicle-end control unit brings about the emission of a request signal. When the request signal is received by the code generator, it automatically transmits back its characteristic response code signal in order to confirm its authorization. If the response code signal is recognized as being authorized, the doors of the motor vehicle are unlocked.
What is referred to as an inquiry/response dialog takes place in an analog fashion if the user is located in the interior of the vehicle and would like to start the vehicle. Given confirmation by the code generator, the immobilizer is then released and the motor vehicle can be started.
Potential car thieves could illegally monitor the signals transmitted wirelessly between vehicle and code generator and illegally reproduce them in order to obtain unauthorized access to the motor vehicle or in order to use the motor vehicle illegally.
For this reason, in a further known anti-theft system (U.S. Pat. No. 4,595,902), not only the code generator but a second transmitter with a different radiation characteristic is also used, the code signal of said second transmitter being, however, not intended to be received by a receiver in the motor vehicle. The electromagnetic field of the second transmitter merely has the purpose of being superposed on the electromagnetic field of the code generator outside the motor vehicle so that the actual code information cannot be obtained by illegally monitoring the field outside the motor vehicle as said information is not present in a separate form.
In another anti-theft system (EP 0 983 916 A1), the propagation time of the emitted signals is measured in the code generator or in the motor vehicle and locked or unlocked only if the propagation time lies within a predefined value range. The rising or falling edges of the signals must be cleanly detectable here for a reliable propagation time measurement to be possible. With superposed signals this is no longer possible.
In a further anti-theft system (EP 0 848 123 A2), when a pushbutton key on the code generator is actuated a start signal in the radio frequency range is transmitted to the motor vehicle, in response to which the inquiry/response dialog is triggered. The transceiver unit in the motor vehicle and code generator transmit in the same frequency range. There is no overlapping of the signals as the signals there otherwise could not be demodulated satisfactorily and separately from one another. Illegal monitoring and illegal reproduction of the signals is thus easily possible. The anti-theft system is therefore easy to bypass.